The present invention is directed to a method for operating a compression ignition engine, and a compression ignition engine itself.
When designing internal combustion engines, there are conflicting requirements between the reduction of different types of emissions like nitrogen oxides (NOx), unburnt hydrocarbons (HC), carbon monoxide (CO) and the reduction of particulate matters (PM). A promising approach to realize highly efficient and low emission combustion is the HCCI-concept (homogeneous charge compression ignition). Here, the ignition of a highly diluted (lean and/or with high rate of exhaust recirculation, EGR) and homogeneous fuel-air-mixture is effected through the temperature increase during the compression stroke close to the upper dead center of the piston. The very dilute fuel-air-mixture allows combustion with extremely low values for nitrogen oxides (NOx).
Auto-ignition of the fuel-air-mixture in the combustion chamber is achieved through a combination of various measures, such as a high geometric compression ratio ε and pre-heating of the charge through suitable measures (for example, pre-heating of the intake air or exhaust gas recirculation, EGR). As according to the HCCI combustion concept the fuel-air-mixture ignites more or less simultaneously in the whole combustion chamber close to top dead center, the combustion event is extremely rapid.
In Diesel-engines, the ignition timing can be easily controlled by the injection time. The control of the ignition time in a HCCI-engine is very demanding.
It is known from the art to ignite the lean and homogeneous fuel-air-mixture through injection of a small amount of a second fuel which tends to autoignite earlier than the first fuel. The choice of start of injection of this secondary fuel can take into account the actual operating condition of the engine. With increasing load of the engine, the amount of the secondary fuel is adjusted.
This concept is known as dual fuel combustion. If the second fuel is injected early and partly pre-mixed for low emissions, this concept is known as dual fuel PCCI or RCCI combustion. If the second fuel is injected in a way that both fuels are mixed homogenously, the concept is known as dual-fuel-HCCI.
The combination of two fuels with different auto-ignition properties allows a much better control of the combustion process. Without such second fuel with different auto-ignition properties, the ignition time can be adjusted through the EGR-rate, that is the percentage amount of recirculated exhaust gas. However, the variation of the external EGR-rate is not a measure with rapid effect, but shows a delayed response.
All known PCCI, HCCI, and RCCI and dual fuel concepts are associated with high HC and CO emissions, as it is well known from literature.
U.S. Pat. No. 6,659,071 shows an internal combustion engine, which can be operated in a PCCI (premixed charged compression ignition) mode. A mixing device forms a mixture of a first fuel with the intake air, a fuel injection device which is capable of injecting a second fuel directly into the combustion chamber, and a control system which controls the injection of the second fuel in such manner. Prior to auto-ignition through the compression of the charge, at least one “control injection” takes place. According to U.S. Pat. No. 6,659,071 it can be foreseen that the main fuel is natural gas and the second fuel is Diesel.
From WO 98/07973 a method to control a PCCI-engine is known, wherein the control of the combustion progress is conducted through measuring an operating state of the engine, which is indicative for the combustion progress. In order to control the start of combustion precisely, the temperature, the pressure, the equivalence ratio and or the auto-ignition properties of the fuel-air-mixture are controlled. Control of the start of ignition and the velocity of ignition is started in such way that basically the complete combustion event takes place within certain crank angle limits, in particular between 20° before the upper dead center through 35° after the upper dead center. This is based on the fact, that the point in time for the beginning of the ignition and the velocity of combustion in a PCCI-engine are depending on the course of temperature, the course of pressure, the auto-ignition properties of the fuel, for example the octane or methane number or the activation energy and the composition of the charge air in cylinder (oxygen content, EGR, moisture, equivalence ratio etc).
U.S. Pat. No. 6,463,907 shows a HCCI-engine and a method to operate such engine, wherein through addition of a secondary fuel, preferably Diesel, the center of combustion is tuned to the preferred crank angle. The desired combustion delay hereby is independent from the combustion duration of the main fuel mixture, which in turn is defined by the EGR rate in connection with the air to fuel ratio. Through the addition of the secondary fuel, the crank angle range, in which combustion takes place, now can be held constant through a wide range of engine speeds. Because of the relatively low burn rates of natural gas after ignition, relatively low EGR rates and high boost pressures are used. Power and speed of subject HCCI-engine are controlled through the air-fuel-mixture or boost pressure.
Also known are approaches to define the ignition timing through the external EGR rate. At high rates of recirculated exhaust gas, the burn rate is slowed down because of the reduced oxygen content.
The control strategy for dual-fuel HCCI engines according to U.S. Pat. No. 6,463,907 is to effect the timing of the spontaneous ignition (auto-ignition) through injection of a fuel with high cetane number, typically diesel, prior to or early in the compression phase. The amount of fuel with a high cetane number added depends on the engine speed and power, and is chosen such that the ignition time is tuned to a suitable crank angle position. The combustion duration is controlled independently through the EGR rate.
To summarize, the conditions for auto-ignition of a lean homogeneous fuel-air-mixture according to the state of the art are controlled by high EGR rates, cooling of the recirculated exhaust, and high geometric compression ratios.